High Surface Area Carbon Materials and Methods for Making Same

ABSTRACT

In a method of making a high surface area carbon material, a precursor organic material is prepared. The precursor organic material is subjected to a first elevated temperature while applying a gaseous purge thereto for a first predetermined time. The precursor organic material is subjected to a second elevated temperature while not applying the gaseous purge thereto for a second predetermined time after the first predetermined time. A high surface area carbon material includes carbon and has a surface area in a range between 3029 m 2 /g to 3565 m 2 /g and a pore volume in a range between 1.66 cm 3 /g and 1.90 cm 3 /g. The high surface area carbon material may be employed in an electrode for a supercapacitor.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.14/042,842, filed Oct. 1, 2013, now U.S. Pat. No. 9,640,333, whichclaims the benefit of U.S. Provisional Patent Application Ser. No.61/709,292, filed Oct. 3, 2012, the entirety of both of which is herebyincorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under agreement No.FA9550-09-1-0150, awarded by the Department of the Air Force. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to methods of making carbon materials and,more specifically, to methods of making high surface area carbonmaterials.

2. Description of the Related Art

One of the outstanding challenges in the field of supercapacitors is toachieve high energy density. To increase energy density in asupercapacitor, electrodes will require higher surface areas withcontrolled pore size distributions, thereby promoting massive chargeaccumulation near the electrode/electrolyte interfaces. The greatestadvantage of supercapacitors over batteries is that they have high powerdensity, enabling them to be charged in fraction of the time required tocharge batteries. Some of the present applications of supercapacitorsinclude: harvesting kinetic energy to store breaking energy in hybridvehicles; and load leveling, i.e. delivering power above the averagevalue when needed and to store excess power when the demand is belowaverage. Improvements in energy density of supercapacitors could lead towidespread use where high energy density along with very high charge anddischarge rates is required, e.g., in such applications as aerospace,industrial, transportation, utility, and consumer electronics.

Supercapacitors are also known as electric double layer capacitors(EDLC) or ultracapacitors. In EDLC, on application of voltage across itselectrodes, charge accumulates in the form of ions at the surface ofelectrodes, forming an electrode-electrolyte double layer. Energydensity of EDLC can be increased by increasing the charge at thesurface, which depends on the accessible surface area to these ions.High surface area electrodes promote massive charge accumulation. Someof the other factors contributing to EDLC energy density are pore size,choice of electrolyte, and electrode materials. Micro pores (with a porediameter of <2 nm) and meso pores (with a pore diameter in the range of2 nm to 50 nm) are important for smooth propagation of solvated ions andhigh electrochemical properties.

Polyacrylonitrile (PAN)-based activated carbons are generally amorphouscarbon with high surface area and good adsorption capacity. Theactivation process for PAN can be achieved by either physical orchemical approaches. Chemical activation tends to generate predominantlymicro-pores with narrow pore size distribution whereas physicalactivation tends to generate predominantly micro and meso-pores withwide pore size distribution.

Current methods of generating carbonaceous materials through activatingPAN materials results in surface areas below 2300 m²/g and relativelylow pore volumes. However in applications such as supercapacitors,batteries, fuel cells, gas absorption and catalysts, surface areas ofgreater than 3000 m²/g would be highly desirable.

Therefore, there is a need for carbon materials exhibiting increasedsurface areas.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is a method of making a high surface area carbonmaterial, in which a precursor organic material is prepared. Theprecursor organic material is subjected to a first elevated temperaturewhile applying a gaseous purge thereto for a first predetermined time.The precursor organic material is subjected to a second elevatedtemperature while not applying the gaseous purge thereto for a secondpredetermined time after the first predetermined time.

In another aspect, the invention is a high surface area carbon materialcomprising carbon and having a surface area in a range between 3029 m²/gto 3565 m²/g and a pore volume in a range between 1.66 cm³/g and 1.90cm³/g.

In yet another aspect, the invention is a supercapacitor that includes afirst electrode and a second electrode. The first electrode includes aconductor layer and a surface layer applied to the conductor layer. Thesurface layer includes a porous carbon material having a surface area ina range between 3029 m²/g to 3565 m²/g and a pore volume in a rangebetween 1.66 cm³/g and 1.90 cm³/g. The second electrode is disposedoppositely from the first electrode and includes a conductor layer and asurface layer applied to the conductor layer. The surface layer includesa porous carbon material having a surface area in a range between 3029m²/g to 3565 m²/g and a pore volume in a range between 1.66 cm³/g and1.90 cm³/g. A membrane separates the 1st electrode from the 2d electrodeand an electrolyte is disposed between the first electrode and thesecond electrode so as to be in chemical communication with the firstsurface layer and the second surface layer.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIGS. 1A-1C are a series of flowcharts demonstrating methods of makinghigh surface area materials.

FIG. 2 is a schematic diagram of a high surface area carbon material.

FIG. 3 is a graph showing an x-ray diffraction measurement of aKOH-activated high surface area carbon powder.

FIG. 4 is a schematic diagram of a portion of a supercapacitor.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. Unless otherwise specifically indicated in the disclosurethat follows, the drawings are not necessarily drawn to scale. As usedin the description herein and throughout the claims, the following termstake the meanings explicitly associated herein, unless the contextclearly dictates otherwise: the meaning of “a,” “an,” and “the” includesplural reference, the meaning of “in” includes “in” and “on.”

As shown in FIG. 1A, in one embodiment of a method for making a highsurface area carbon material, a precursor is prepared 100. Typically, anorganic polymer such as polyacrylonitrle-co-methacrylate (PAN) isemployed. (In one example, homopolymer PAN is used and in anotherexample, copolymer PAN is used. Examples of copolymers include but arenot limited to polyacrylonitrile-co-methacrylic acid,polyacrylonitrile-co-methyl acrylate, polyacrylonitrile-co-itaconicacid, polyacrylonitrile-co-itaconic acid-co-methacrylic acid,polyacrylonitrile-co-methyl methacarylate.) In one example, a PAN powderis used and in another example, a PAN film is used. A first precursorstabilization with an air purge 102 is performed. A second precursorstabilization without the air purge 104 is performed. In one example,both the first precursor stabilization step 102 and the second precursorstabilization step 104 were performed at 285° C. In the first precursorstabilization with an air purge 102, air is introduced into the reactionchamber and in the second precursor stabilization without the air purge104 no air is added to the reaction chamber, but any gasses that formduring this step are allowed to vent out of the chamber.

In one experimental embodiment, 300 mg of PAN was dissolved in 30 mL DMF(in other examples, one several other solvents may be used, such as DMAcor DMSO) at 80° C. for one hour. The material was cast as a film on ahot (80° C.) glass substrate for about 12 hours at a 15 psi vacuum. Thefilm was separated from the glass and dried at 80° C. for 48 hours,resulting in a film having a thickness of about 25 In one embodiment,the first stabilization step included subjecting such a PAN film to anair purge for 10 hours and then a second stabilization step subjectingthe PAN film to an environment without an air purge for 6 more hours,both at 285° C. In another embodiment, the first stabilization stepincluded subjecting such a PAN film to an air purge for 16 hours andthen a second stabilization step subjecting the PAN film to anenvironment without an air purge for 6 more hours, both at 285° C. Inyet another embodiment, the first stabilization step included subjectingsuch a PAN powder to an air purge for 16 hours and then a secondstabilization step subjecting the PAN powder to an environment withoutan air purge for 6 more hours, both at 285° C. The thus stabilizedmaterials were then soaked in 6M KOH for 24 hours and the resultingKOH-soaked materials were activated at 800° C. for 1 hour in an inert(Ar) environment (in which the heating rate from room temperature to800° C. was 5° C. per minute). The resulting activated materials werewashed in boiling water four times and dried at 80° C. in a vacuum ovenfor 24 hours. The surface area of this carbon material was measured bynitrogen gas absorption in a range from 3029 m²/g to 3565 m²/g. In otherexperimental embodiments, the activated carbon materials were preparedinto two different forms (film and powder). PAN films were stabilized atdifferent residence time to investigate the effect on the surface areaand pore structure, further on the resulting electrochemical properties.The surface area and pore structure analysis for the activated carbonmaterials were done by nitrogen gas adsorption-desorption at 77K usingASAP 2020 (Micromeritics Inc). For the analysis, the activated carbonmaterials were degassed at 90° C. for 16 hours. BET (Brunauer, Emmet,and Teller) analysis for surface area and density functional theory(DFT) analysis for pore volume and pore size distribution wereconducted.

In one embodiment, the stabilized precursor material is soaked in a KOHsolution (or other ionic solution) for a predetermined amount of time(such as 24 hours) to impregnate the stabilized precursor material withKOH ions 106. The material is then activated 108 by subjecting it to anelevated temperature (e.g., 800° C.) for an amount of time (e.g., 1hour) to remove volatile components from the now-carbonized material.The high surface area carbon is then washed 110 (e.g., in boiling water)and dried (e.g., at 80° C. in a vacuum for 24 hours). At this stage, thematerial is now high surface area carbon.

As shown in FIG. 1B, the precursor material is carbonized 112 withoutKOH impregnation and then activated 114 by subjecting it to an elevatedtemperature (e.g., 800° C.) for an amount of time (e.g., 1 hour).

A resulting carbon structure 200 is shown schematically in FIG. 2 and anx-ray diffraction measurement 300 of a KOH-activated high surface areacarbon powder is shown in FIG. 3. As can be seen, this measurement showsno diffraction 20 peak corresponding to graphite [0002] spacing, whichindicates that there is no substantial graphene stacking in thestructure.

In the embodiment shown in FIG. 1C, carbonaceous powder was also made bystabilizing PAN powder at 285° C. (heating 1° C./min.) for 16 hours inthe presence of air 102 and 6 hours after air purging stopped 104.Stabilized powder was carbonized 112 at 1100° C. (heating from roomtemperature to 1100° C. at 5° C./min.) in the presence of argon. Suchcarbonized PAN powder demonstrated a BET surface area 2298 m²/g. Thiscarbonaceous material did not demonstrate the presence of micro pores(<2 nm), and the majority of pores were in the range of 2 nm to 50 nm(meso pores).

As shown in FIG. 4, the high surface area carbon 410 produced by thismethod can be used in electrodes 402 employed in supercapacitors 400 andother applications requiring high surface area materials. In oneembodiment of the supercapacitor application, the electrodes 402 includea layer 408 of 0.75 mg of carbon nanotubes (CNTs), a layer 410 of 4 mgactivated PAN powder mixed with 1.0 mg of CNTs, a layer 412 of 0.25 mgof CNTs, and a layer of cellulose filter paper 414. The electrodes 402are disposed oppositely from each other and an electrolyte 420 (such asa KOH solution) is disposed between the electrodes 402.

While the as-prepared activated PAN films were used directly aselectrodes for a supercapacitor cell 400, the activated PAN powder-basedelectrode 402 was prepared using CNTs to improve electrical conductivityand to improve the structural integrity of the activated PAN powder 410.The CNTs were sonicated in DMF for 24 hours at the concentration of 1mg/300 mL and the activated PAN powder was mixed with CNT dispersion(activated PAN powder:CNT=4:1 by weight) by sonication for 30 minutes.Then, the dispersion was filtered using cellulose filter paper (1 μmpore size). Before use as an electrode, the activated PAN powder/CNTfilm was vacuum dried at 100° C. for 4 days.

In one experimental embodiment, the prepared electrodes were separatedby a non-conducting porous polypropylene membrane (Celgard 3400,0.117×0.042 μm) and sandwiched between nickel current collectors. Theelectrodes and membrane were soaked in the electrolyte solution for 30min prior to cell assembly. For the activated PAN film-based electrode,either aqueous KOH (6 M) or an ionic/organic (BMIMBF₄/AN) liquid wereused as an electrolyte, whereas ionic liquid EMIMBF₄ was used for theactivated PAN powder/CNT-based electrode embodiment.

In experimental embodiments, chemical activation using KOH was adoptedfor various PAN materials (film and powder), leading to the average poresize of 2.5 nm with surface area exceeding 3500 m²/g. In addition,electrolytes for supercapacitors have a wide operating voltage range andremain stable at high temperature. Ionic liquids provided wide voltagerange and stability at higher temperature than aqueous electrolytes.Therefore, different types of ionic liquid electrolytes (BMIMBF₄ andEMIMBF₄) were also used in these embodiments, along with KOH aqueouselectrolyte. The electrode made from the high surface area carbonaceousfragments exhibited highest density (which was measured to be in a rangeof 40 Wh/kg to 100 Wh/kg when using EMIMBF₄).

In various embodiments, high surface area carbon materials exhibitedsurface areas in the range of between 3029 m²/g to 3565 m²/g and porevolumes of between 1.66 cm³/g to 1.90 cm³/g, with micro pore percentagesof between 31% to 38% and meso pore percentages of between 62% to 68%.

The above described embodiments, while including the preferredembodiment and the best mode of the invention known to the inventor atthe time of filing, are given as illustrative examples only. It will bereadily appreciated that many deviations may be made from the specificembodiments disclosed in this specification without departing from thespirit and scope of the invention. Accordingly, the scope of theinvention is to be determined by the claims below rather than beinglimited to the specifically described embodiments above.

What is claimed is:
 1. A high surface area carbon material comprising carbon and having a surface area in a range between 3029 m²/g to 3565 m²/g.
 2. The high surface area carbon material of claim 1 having a pore volume in a range between 1.66 cm³/g and 1.90 cm³/g.
 3. The high surface area carbon material of claim 1 having a 38% micro pore volume and a 62% meso pore volume.
 4. A supercapacitor, comprising: (a) a first electrode including: (i) a conductor layer; and (ii) a surface layer applied to the conductor layer, the surface layer including a porous carbon material having a surface area in a range between 3029 m²/g to 3565 m²/g and a pore volume in a range between 1.66 cm³/g and 1.90 cm³/g; (b) a second electrode disposed oppositely from the first electrode and including: (i) a conductor layer; and (ii) a surface layer applied to the conductor layer, the surface layer including a porous carbon material having a surface area in a range between 3029 m²/g to 3565 m²/g and a pore volume in a range between 1.66 cm³/g and 1.90 cm³/g; and (c) an electrolyte disposed between the first electrode and the second electrode so as to be in chemical communication with the first surface layer and the second surface layer.
 5. The supercapacitor of claim 4, wherein the conductor layer of the first electrode and the second electrode comprises a metal. 